Varistor elements are assembled in electronic devices, such as computers and cellular phones, to protect electronic circuits and elements included in them against abnormally high voltages created by static electricity, noise, and the like.
Such varistor elements utilize varistors (varistor material) that exhibit a high electrical resistance at a low applied voltage, allowing the flow of only slight current, while exhibiting a drastically reduced electrical resistance at a high applied voltage to allow for the flow of a large amount of current. Thus, the varistors have a nonlinear resistance. Among these varistors, a zinc oxide-based varistor (zinc oxide-based varistor material) is frequently used because it can easily achieve the desired characteristics. The zinc oxide-based varistor contains zinc oxide as a main component along with other oxides added thereto.
Such a varistor element, which uses the zinc oxide-based varistor material, specially, the sintered body for the zinc oxide-based varistor produced by sintering this material, is assembled and incorporated in an electronic circuit. For this reason, even though current flows through part of the electronic circuit due to excessive voltage caused by static electricity, noise, or the like, the varistor element can suppress the flow of a large amount of current through a target part of the electronic circuit as well as a target element therein.
On the other hand, a space occupied by the varistor element could be an obstacle for the miniaturization of electronic devices.
For example, one means of solving this problem is seen in Patent Document 1. which describes how an electrostatic discharge (ESD) protection device, such as the varistor element, can be integrally formed as a low-temperature co-fired ceramic (LTCC) with the electrodes in a multilayer substrate.
The electrodes in contact with the LTCC are desirably made using silver (Ag) with a low electrical resistance. However, when heated to a high temperature, for example, 1,000° C. or higher, silver could degrade in performance as the electrode due to its oxidization and the like.
For this reason, Patent Documents 2 to 6 describe a method for forming a multilayer substrate with the varistor element (ESD protection device) that involves firing a laminated body including mixed-raw-material sheets (green sheets) of varistor material or the like with electrode material of silver formed at the surface of each sheet at a temperature ranging approximately from 850° C. to 950°.
Patent Document 1: WO 2009/136535 A
Patent Document 2: JP 2010-238882 A
Patent Document 3: JP 2007-5499 A
Patent Document 4: JP 9-312203 A
Patent Document 5: JP 2012-114443 A
Patent Document 6: JP 2005-97070 A